Structure code to run before any application starts

Maybe you can explain exactly what you are trying to accomplish.

Nerves does something like this with the Shoehorn application, it ALWAYS starts, so even if other applications crash, you always have something running.

I’m not exactly sure how it accomplishes this, but you could dig into the code. Shoehorn — shoehorn v0.9.3

But maybe there’s another way to accomplish what you need to do.

Good idea, this could be an example of the XY problem. I am trying to store some DB rows in :persistent_term and want to guarantee that the rows will be stored in :persistent_term before any of the applications start.

And thanks for the link, will take a look!

If it wasn’t an umbrella application, you could just use a supervision tree, and have your first process insert the data, but assuming you don’t want to refactor everything Shoehorn might be exactly what you need.

At the same time, the idea of a global state being shared across all of your applications is kind of frightening.

What exactly is the use case?

Couldn’t you create a new application (that all of the other applications depend on), whose supervision tree contains a GenServer that inserts the rows in :persistent_term?

In addition to @axelson’s excellent suggestion, do also have a look at GenServer’s handle_continue.

I have used the Module @on_load callback to ensure persistent terms are set up the first time the wrapper module is used. That allowed me to avoid any concerns about app startup processes.

That’s an interesting approach, but I am not able to get it to work with modules that represent a schema. I keep getting the error: __MODULE__.__schema__/1 is undefined (function not available). Do you know what might be causing it?

I suspect Ecto’s schema macro is setting an @on_load callback and yours overrides it. If you could find the right call, your callback could call Ecto’s

Thanks for the quick reply! Turns out I was not returning :ok like I was supposed to from the on_load function. Returning :ok solved the issue.

This approach looks promising. I am going to play around with it and report back.

Oh, good because I just grepped the ecto code and “on_load” does not appear, so my suspicion was all wrong

I think my first attempt forgot that :ok too, so I can relate. I hope it works out for you.